This invention relates to electrode materials useful in secondary lithium batteries.
Two classes of materials have been proposed as anodes for secondary lithium batteries. One class includes materials such as graphite and other forms of carbon, which are capable of intercalating lithium. While the intercalation anodes generally exhibit good cycle life and coulombic efficiency, their capacity is relatively low. A second class includes metals that alloy with lithium metal. Although these alloy-type anodes generally exhibit higher capacities relative to intercalation-type anodes, they suffer from relatively poor cycle life and coulombic efficiency.
The invention provides electrode compositions suitable for use in secondary lithium batteries in which the electrode compositions have high initial capacities that are retained even after repeated cycling. The electrode compositions, and batteries incorporating these compositions, are also readily manufactured.
To achieve these objectives, the invention features an electrode composition that includes an electrode material consisting essentially of at least one electrochemically inactive elemental metal and at least one electrochemically active elemental metal in the form of an amorphous mixture at ambient temperature. The electrode material is essentially free of intermetallic compounds. The mixture of elemental metals remains amorphous when the electrode composition is incorporated into a lithium battery and cycled through at least one full charge-discharge cycle at ambient temperature. Preferably, the mixture remains amorphous after cycling through at least 10 cycles, more preferably at least 100 cycles, and even more preferably at least 1000 cycles.
An xe2x80x9celectrochemically active elemental metalxe2x80x9d is a metal that reacts with lithium under conditions typically encountered during charging and discharging in a lithium battery. An xe2x80x9celectrochemically inactive elemental metalxe2x80x9d is a metal that does not react with lithium under those conditions.
An xe2x80x9camorphous mixturexe2x80x9d is a mixture that lacks the long range atomic order characteristic of crystalline material. The existence of an amorphous mixture can be confirmed using techniques such as x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry.
When incorporated in a lithium battery, the electrode composition preferably exhibits (a) a specific capacity of at least about 100 mAh per gram of active metal for 30 full charge-discharge cycles and (b) a coulombic efficiency of at least 99% (preferably at least 99.5%, more preferably at least 99.9%) for 30 full charge-discharge cycles when cycled to realize about 100 mAh per gram of active metal of the composition. Preferably, this level of performance is realized for 500 cycles, more preferably for 1000 cycles.
In another preferred embodiment, the electrode composition, when incorporated in a lithium battery, exhibits (a) a specific capacity of at least about 500 mAh per gram of active metal for 30 full charge-discharge cycles and (b) a coulombic efficiency of at least 99% (preferably at least 99.5%, more preferably at least 99.9%) for 30 full charge-discharge cycles when cycled to realize about 500 mAh per gram of active metal of the composition. Preferably, this level of performance is realized for 200 cycles, more preferably for 500 cycles.
The electrode composition can be in the form of a thin film or a powder. Thin films can be prepared using a number of techniques, including sputtering and melt spinning. Examples of suitable electrochemically active elemental metals include aluminum, silicon, tin, antimony, lead, germanium, magnesium, zinc, cadmium, bismuth, and indium. Examples of suitable electrochemically inactive elemental metals include Group IB through Group VIIB elemental metals, as well as group VIII and rare earth elemental metals. Specific examples include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, La, Hf, Ta, W, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, and Sm. Of this group, molybdenum, niobium, tungsten, tantalum, iron, nickel, manganese, and copper are preferred.
Lithium batteries including the above-described electrode compositions may be used as power supplies in a variety of applications. Examples include power supplies for motor vehicles, computers, power tools, and telecommunications devices.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.